In fiber-optic communication systems, digital data are transmitted from sending stations to receiving stations by means of optical pulses propagating in optical fibers. Each of the fibers thus carries (propagates) a digital data stream (sequence) of optical pulses typically formed by pulse code modulation, or other kind of pulse modulation, of a continuous optical wave called the "carrier". This carrier typically is a monochromatic continuous-wave optical beam of radiation, as supplied by a laser oscillator source of light having a wavelength in the near infra-red, for example, a wavelength of about 1.5 .mu.m (1.5 micrometers). During each predetermined time interval ("slot") the amplitude of the carrier wave is modulated in accordance with one bit of digital information, so that during each such time slot the carrier wave contains a pulse of standard height or no pulse at all, in order to represent a binary digital "1" or "0", respectively, for example. The thus pulse-modulated optical beam propagates through fiber segments (fiber transmission lines), and each of such fiber segments extends from a sending station to a receiving station. At the receiving station, typically the fiber segment terminates in a signal regenerator device to restore the signal--i.e., to reduce noise and to restore the amplitude of the pulses--to a standard level for further propagation in fiber or for other uses.
As the technology progresses, the desired data rates and hence pulse repetition rates increase in order to increase the data handling capacity of the system without concomitantly increasing the required number of relatively costly fiber transmission lines. As these pulse repetition rates are increased to values of 8 GHz (8 gigahertz), corresponding to data rates of 8 Gb/s (8 gigabits/second), or more, a major problem arises because of the phenomenon of optical dispersion by the transmission medium, to wit, the fiber material, particularly in case of a carrier wavelength of about 1.5 .mu.m. Dispersion is the inherent property of any transmission medium that different optical frequencies (different wavelengths) propagate through the medium at different velocities. Thus, since even a purely monochromatic (single optical frequency) beam of continuous-wave optical radiation is not purely monochromatic after it is modulated, e.g. pulsed, the phenomenon of dispersion of any pulsed optical beam occurs during propagation through fiber. As a result of this dispersion in fiber material, when a stream of digital optical pulses is introduced into one end (input end) of an optical fiber segment, it emerges from the other end (output end) as a stream of optical output pulses that are degraded, i.e., are spread out ("smeared") in time and space. As the length of the fiber segment is increased, the effects of dispersion accumulate, whereby the smearing of the pulses becomes more severe. Thus, as the length of a fiber segment is increased beyond a threshold, the output pulses become unrecognizable (unrestorable) as such. That is, it becomes impossible to determine the transmitted information--i.e., to decide at the output end of the fiber which of the time slots had been designated for carrying a pulse and hence are supposedly representing binary digital "1" or which of the time slots are not carrying a pulse and hence are supposedly representing binary digital "0".
As is known from Fourier transform theory, the product of the half-width of the optical Fourier spectrum of an optical pulse and the width of the pulse (measured in units of time) is equal to unity or more, depending upon the pulse shape (profile). Thus for a given pulse shape, as the pulse repetition rate and hence the data rate increases, the width of each pulse must decrease, and hence the half-width of optical Fourier spectrum must increase. Consequently, the spread of optical frequency Fourier components increases as the data rate increases, whereby the spread of optical propagation velocities due to the fiber material dispersion property increases, and hence for a given fiber span (length) the degradation of signal becomes more severe. Even if the pulse shape is such that its optical spectrum is Fourier transform limited, and thus minimizes the effects of dispersion and hence maximizes the fiber span, the degradation of signal by dispersion in fiber can still be a major problem.
At the same time the fiber disperses a propagating signal it also absorbs light by scattering or other phenomena whereby signal level is undesirably reduced. Thus, although the dispersion problem could be alleviated by using a carrier having a wavelength that undergoes less dispersion, such as a carrier having a wavelength of about 1.3 .mu.m, the absorption phenomena would then impose an even more serious limitation on fiber span.
In a paper "Use of Chirp Pulses to Improve the Pulse Transmission Characteristics in a Dielectric Optical Waveguide" by T. Suzuki, published in Electronics and Communications in Japan, Vol. 59-C, No. 3, pp. 117-125 (1976), it was proposed that a chirped pulse technique be used to modulate the optical frequency of the pulse stream by passing it through a lithium niobate crystal whose refractive index was being frequency-modulated (equivalent to time-dependent phase-modulation) by means of an applied external a.c. electric field of frequency in sychronism with the pulse repetition rate, whereby the optical frequency of the carrier wave varied monotonically across the pulse from its leading (front) edge to it trailing (rear) edge; thus the pulse was chirped. Consequently, the pulse was compressed by dispersion during its transmission through an initial portion of optical fiber: the fiber dispersion slows the propagation of the front portion of the chirped pulse and speeds the propagation of the rear portion thereof. In this way, the chirping of the pulses served to compensate the effects of dispersion, and the total length of fiber segment that was capable of propagating the pulse in a recognizable shape (form) was increased; that is, chirping the pulses increased the span. However, this technique has several shortcomings including a significant lowering of signal level (modulation loss) while propagating through the electro-optic crystal, such as lithium niobate, as well as the requirement of relatively large applied electric fields in the crystal and hence the undesirable consumption of unduly large amounts of energy required to produce a significant chirp and hence a significant increase in the fiber span--i.e., an increase of at least about 50% (a factor of at least about 1.5). As pulse repetition rates increase, and hence as the frequency of electric fields applied to the electro-optic crystal increase, these shortcoming become increasingly severe, so that this technique is impractical for pulse repetition rates equal to or greater than about 1 GHz. Another technique for chirping and hence compressing optical pulses involves the chirping, by self-phase modulation in fiber itself, of intense optical pulses while propagating through the fiber, as described in U.S. Pat. No. 4,588,957, issued on May 13, 1986 to Balant et al. entitled "Optical Pulse Compression Apparatus and Method". However, for pulse repetition rates of 10 GHz and higher, that requires an energy per pulse of more than 10 picojoule per pulse, which is undesirably high.
Therefore it would be desirable to have a technique for increasing the span of fiber capable of propagating optical pulses having a carrier wavelength in the near infra-red and a pulse repetition rate of about 8 GHz or more that avoids the shortcomings of prior art.